This link was especially strong in patients without dementia, according to the study in the current issue of the Journal of Parkinson's Disease.

"About 30 percent of persons with [Parkinson's disease] suffer from cognitive impairment and dementia, and dementia is associated with nursing home placement and shortened life expectancy," study author Dr. Amie Peterson, of the Oregon Health and Sciences University, said in a journal news release.

"We know mild cognitive impairment may predict the future development of dementia," she added. Preventing the development of dementia in these patients may potentially improve rates of illness and death related to Parkinson's disease, Peterson suggested.

However, the study doesn't show whether low vitamin D dulls thinking or if the opposite is true -- that people with more advanced Parkinson's disease get less sun exposure because of their limited mobility and have lower levels of vitamin D as a result. The study also did not ask if patients were taking vitamin D supplements. While the study showed an association between vitamin D levels and thinking problems, it did not prove a cause-and-effect link.

Vitamin D is absorbed by the body from sunlight. It is also found in foods such as fatty fish and in supplements.